Meal plans were designed to decrease daily energy intake by approximately 500 calories/day, with a goal of 10 percent weight loss over six months. Participants were randomly assigned to receive a protein supplement (50 grams/day of whey protein isolate) or a carbohydrate supplement (50 g/day maltodextrin). The higher-protein diet provided approximately 30 percent of calories from protein, 40 percent of calories from carbohydrate and 30 percent of calories from fat; the higher-carbohydrate diet provided approximately 18 percent of calories from protein, 52 percent of calories from carbohydrate and 30 percent of calories from fat. Dual-energy x-ray absorptiometry (DXA) and magnetic resonance imaging (MRI) were used to measure muscle and fat changes during the intervention.

There were no differences in strength, balance or physical performance between the protein and carbohydrate groups. However, when data from groups were combined, strength decreased, but balance and physical performance improved from baseline for all subjects. The authors said in order to see a significant effect on physical function, some aspects of the study design may have needed to change, such as having a bigger sample size, providing more whey protein, inducing a greater weight change, using more sensitive performance measures or investigating a population with compromised physical function.

The authors said the improved maintenance of muscle seen in this population indicates that a higher-protein intake can be important during weight loss. This whey protein supplementation study adds to the published literature on the benefits of a higher-protein diet on body composition during weight loss. The study also provides excellent preliminary data for future clinical trials to determine the most beneficial combination of a higher protein diet, weight loss and exercise to improve body composition and physical function in older women.